A mood stabilizer is a psychiatric medication used to treat mood disorders characterized by intense and sustained mood shifts, which is not the same as "feeling good one minute and then bad the next." The most common is bipolar disorder, where mood stabilizers suppress swings between mania and depression, and these drugs are also used in borderline personality disorder. Most mood stabilizers are anticonvulsants, with the important exception of lithium, which is the oldest and best known mood stabilizing drug.

Most so-called mood stabilizers are purely antimanic agents, meaning that they are effective at treating mania and mood cycling and shifting, but are not effective at treating depression. The principal exceptions to that rule, because they treat both manic and depressive symptoms, are lamotrigine and lithium carbonate. While an antimanic agent such as valproic acid or carbamazepine cannot treat depression directly as the former two drugs can, it is widely thought to help ward off depression in bipolar patients by keeping them out of mania and thus preventing their moods from cycling.

Nevertheless, an antidepressant is often prescribed in addition to the mood stabilizer during depressive phases. This brings some risks, however, as antidepressants can induce mania, psychosis, and other disturbing problems in bipolar patients -- particularly when taken alone, but sometimes even when used with a mood stabilizer. It should be noted that antidepressants' utility in treating depression-phase bipolar disorder is unclear.

Drugs commonly classed as mood stabilizers include:

• Lithium carbonate —- Lithium is the 'classic' mood stabilizer. The first mood stabilizer to be approved of the United States Food and Drug Administration, and still popular in treatment. Therapeutic drug monitoring required. Monitor blood lithium levels (therapeutic range: 0.6 or 0.8-1.2 mEq/L) and look for signs and symptoms of toxicity (such as nausea, vomiting, diarrhea, ataxia). See also lithium orotate, another lithium salt.
• Valproic acid (Depakene ® ), divalproex sodium (Depakote ® ), and sodium valproate (Depacon ® ) — Available in extended release form. Can be very irritating to the stomach, especially when taken as valproic acid. Liver function and CBC should be monitored. [Therapeutic drug monitoring is required.
• Lamotrigine (Lamictal ® ) — Particularly effective for bipolar depression. Monitor for signs and symptoms of Stevens-Johnson syndrome, very rare but can be fatal.
• Carbamazepine (Tegretol ® ) — CBC should be monitored; can lower white blood cell count. Therapeutic drug monitoring is required. Only very recently (as of 2005) FDA-approved for bipolar disorder, but widely used for many years.
• Gabapentin (Neurontin ® ) — Not FDA approved for bipolar disorder. Recent scientific studies suggest it is not an effective treatment, however many psychiatrists continue to use it.
• Oxcarbazepine (Trileptal ® ) — Not FDA approved for bipolar disorder.
• Topiramate (Topamax ® ) — Not FDA approved for bipolar disorder.

Sometimes mood stabilizers are used in combination, such as lithium with one of the anticonvulsants.

Many atypical antipsychotics also have mood stabilizing effects and are thus commonly prescribed even when psychotic symptoms are absent. It is also conjectured that Omega-3 fatty acids may have a mood stabilizing effect. However, more research is needed to verify this (a multi-year study of this is now being carried out as of 2001).